present invention relates, in general, to the field of power MOS switches. More particularly, the present invention relates to a power MOS switch having a control circuit which includes current limit, power dissipation limit and thermal limit circuits wherein these protection circuits retain their capability after loss of ground connection to the control circuit.
There have been previously described power MOS switching circuits wherein a CMOS control circuit is used to control a high power MOS device. The CMOS control circuit includes current limit, power dissipation limit and thermal limit portions which prevent the destruction of the power MOS device. Such a circuit is commercially available from Motorola, Inc. and bears Part No. XPC1500. While this control circuit provides certain protection to the power MOS device under normal conditions these protective circuits cease to function or their function is degraded when the ground connection to the control circuit is lost. When this control circuit loses its ground connection, the IC portion of the device will float to approximately the supply voltage causing the power MOS to turn on partially. Since none of the device protection functions are then operational, the power MOS device can then go to thermal destruction.